Counting how many people live in the most populous nation on earth was never going to be easy, but in a country where a second child is more often illegal, the lonely life of the 21st century census-taker can feel more like mission impossible.

Three census takers prepare to work a Tianjin residential block.

Qiu Xiaoyan, a census-taker for the Ziwei Huayuan residents' committee in Kangjian residential district in Xuhui district in Shanghai, is getting ready for another day of knocking on several of 200 doors in three residential compounds.

"Nowadays residents are more welcoming toward younger people than those old ayi (aunties) that come to mind when you think of these sorts of things," says the 26-year-old who began working at the residents' committee a year ago.

Qiu has been working two weeks on the census that officially only kicked off five days ago.

"It was impossible to get all the work done in 10 days," Qiu, 26, says. "So we had to start much earlier and find everything that might be helpful to us in fulfilling the task on time".

The census office at her residents' committee collated information from the police, the population and family planning commission and property management companies to assist them with registration, Qiu explains.

Qiu averages 20 households a day working 9 am to late at night, sometimes midnight.

Qiu feels frustrated over wasting too much of her time playing cat-and-mouse with nonconformists and their various excuses: no need to attend, no time to attend, asking her to fill out the form for them or most often simply refusing to open the door even when at home.

It's a high stakes game.

In making an early public announcement that those who registered their illegal extra child would receive "reduced" fines, authorities actually appeared to believe this would encourage participation in the census - rather than alerting migrant workers to head for the hills.

For those migrant workers caught harboring a second child, a hefty fine and who-knows-what other punishment makes Qiu the census-taker an especially unwelcome guest at their home.

Somebody's ringing the bell

It's not just migrant workers who have reason to fear Qiu.

Pioneering members of China's nascent civil society are mindful of the seven "census-takers" who knocked at the door of banned book author Xie Chaoping's rented home in Beijing.

When his wife Li Qiong answered, they asked her if Xie lived there.